This invention relates to the detection of malfunctions that may occur in exhaust gas recirculation (EGR) systems of internal combustion engines. More specifically, the invention pertains to a method of, and apparatus for, diagnosing an automotive engine EGR system for possible malfunctions by sensing the temperature of the recirculating exhaust gas with a thermistor or like temperature sensor. The method and apparatus of the invention are particularly well suited for use with EGR systems of manual transmission, rather than automatic transmission, vehicles.
EGR constitutes one of the long familiar methods of cleaning the automotive exhaust gas for the reduction of air pollution. In its simplest form the EGR system comprises a valve, commonly referred to as the EGR valve, for opening and closing a special passageway connecting the exhaust manifold with the intake manifold. The EGR valve is vacuum operated to open when the throttle is opened, with the result that part of the exhaust gas passes into the intake manifold. Such recirculation of exhaust gas serves to reduce the combustion temperature and to lower the formation of nitrogen oxides.
The performance of the engine itself is hardly affected even if the exhaust gas does not recirculate because of the malfunction of the EGR valve. In all likelihood, therefore, the vehicle operator will not notice such malfunctions unless means are incorporated with the EGR system for constantly checking it for possible malfunctions and for giving an alarm upon detection of a malfunction.
A well known example of such checking means is a temperature sensor provided on the exhaust gas passageway from exhaust manifold to intake manifold. Whether the exhaust gas is recirculating as required or not is ascertained from the temperature of the exhaust gas passageway. Japanese Utility Model Publication No. 52-31886 is hereby cited as a typical prior art device employing the temperature sensor. The prior art device additionally comprises a serial connection of a switch responsive to the temperature sensor, another switch responsive to engine speed, and an alarm. The alarm is actuated when the exhaust gas temperature drops below a certain limit for some malfunctions of the EGR system when the engine speed is high.
There is one consideration that must go into the design of diagnostic systems with use of thermistors as the exhaust gas temperature sensors. The thermistor, as is well known, is a resistive circuit element whose resistance varies in a known manner with the temperature. Its response is not very good; the response time may be as long as several seconds for correct measurement of the temperature.
It might, therefore, be contemplated to diagnose the performance of the EGR system by the indications of the thermistor only when the prescribed engine operating conditions for justifying exhaust gas recirculation lasts for a preassigned length of time. The greatest justification for exhaust gas recirculation comes at the time of hard acceleration immediately following vehicle startup, with the throttle opened to more than a certain degree and with the engine speed rapidly building up. During such hard acceleration, however, the ER system operates either continuously or discontinuously depending upon whether the engine is equipped with an automatic or a manual transmission. The automatic transmission automatically shifts to higher gears as the vehicle operator keeps stepping on the accelerator pedal. Exhaust gas will continuously recirculate during such automatic upshifting. Malfunction diagnosis with the thermistor is possible during the continuous gas recirculation.
However, with the manual transmission, the vehicle operator usually makes a rapid succession of manual upshifts as the vehicle speed builds up by the depression of the accelerator. Exhaust gas does not recirculate during the manual upshifts. If the intervals of the upshifts are each shorter than the preassigned length of time, no diagnosis will be made during the startup period. As far as the applicant is aware, there has been suggested no satisfactory method nor apparatus for effectively detecting malfunction in the EGR systems on manual transmission vehicles in the face of the rapid succession of manual upshifts.